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# effective compression ratio

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# effective compression ratio

06-12-2017, 01:37 AM
#1
On The Tree

Join Date: Nov 2016
Location: switzerland
Posts: 131
effective compression ratio

i found this:

"So the big question becomes, how much boost do we gain for X amount of compression? The best method we have found is to calculate the effective compression ratio (ECR) with boost. The problem is that most people use an incorrect formula that says that 14.7psi of boost on a 8.5:1 motor is a 17:1 ECR. So how in the world do people get away with this combination on pump gas? You can't even idle down the street on pump gas on a true 17:1 compression motor. Here's the real formula to use:
sqrt((boost+14.7)/14.7) * CR = ECR
sqrt = square root
boost = psi of boost
CR = static compression ratio of the motor
ECR = effective compression ratio
So our above example gives an ECR of 12.0:1. This makes perfect sense, because 12:1 is considered to be the max safe limit with aluminum heads on pump gas, and 15psi is about as much boost as you can safely run before you at least start losing a significant amount of timing to knock. Of course every motor is different, and no formula is going to be perfect for all combinations, but this one is vastly better than the standard formula (which leaves out the square root)."

any thoughts?

06-12-2017, 11:56 AM
#2

Join Date: Dec 2008
Location: Santa Ana, CA. USA
Posts: 2,075
Density Ratio

Hi Dian, SORRY as your report is not correct.

I asked the same question to Bob Keller when a still worked for AiResearch more than forty years ago.

His Answer : Use Air Density x C.R. to get the Dynamic Compression value.

The 1 BAR increase (14.7) is 1.5 if 100% efficient, now DEGRADE this value using temperature in Rankine AND you will find a 1.3-1.35 often with an efficient charge air cooling system.

Thus your 1 Bar with 8:1 would become 10.4 as common.

Lance

06-13-2017, 03:43 AM
#3
On The Tree

Join Date: Nov 2016
Location: switzerland
Posts: 131

i came across this by chance and it seem to be an ad hoc model, that, if it check o.k. agains reality, might be usefull. i was jus wondering if anybody has seen/used that.

06-17-2017, 10:44 AM
#4
TECH Senior Member

Join Date: Feb 2009
Location: Detroit
Posts: 6,540

How is the information from this calculation valuable?

06-17-2017, 11:09 AM
#5

Join Date: Dec 2008
Location: Santa Ana, CA. USA
Posts: 2,075
Turbo C.R.

Hi "ssT", I bought my first turbos from AK Miller, there I met Big Bill Edwards.
He was the first to set world records with a "turbo" SBC, Land Speed Records
This was FORTY FIVE years ago.
I was the first to "turbo" a 351-C in the Ford Pantera (1973)

This was the topic we spoke, the C.R. (dynamic) of an engine when in boost.

We contacted Bob Keller at AiResearch.

His answer is STATED above !

Those here at LS-1 Tech ALWAYS ask "how much boost on what fuel"

THUS the NEED.

Lance

06-17-2017, 11:22 AM
#6
TECH Senior Member

Join Date: Feb 2009
Location: Detroit
Posts: 6,540

CR and DCR are independent of how much air makes it into the cylinder though. You need RPM, temp, and several other things to get any useful information. Maybe you mean charge ratio?

06-17-2017, 02:06 PM
#7
Old School Heavy

Join Date: May 2010
Location: Florida
Posts: 8,786

Originally Posted by smokeshow
CR and DCR are independent of how much air makes it into the cylinder though. You need RPM, temp, and several other things to get any useful information. Maybe you mean charge ratio?
Agreed. Volumetric efficiency is missing from the equation, just like it is from the n/a DCR formula.

06-18-2017, 08:10 PM
#8

Join Date: Dec 2008
Location: Santa Ana, CA. USA
Posts: 2,075
Pantera EFI EAP Question

Hi ALL, SURE and I agree, this is WHY I use EAP.

The OP STATED a "general" question WITH QUICK FOUNDATION !

MY ANSWER IS CORRECT with the "limited" a limited (no disregard) QUESTION.

ALL HERE (MOST) HAVE NO RESPECT with the USE of EMPERICAL DATA !

THE LS-TECH SPELL CORRECTION IS ALSO IGNORANT "my loss" AND to ALL MEMBERS ! @ Lance (*!*)

Lance

06-19-2017, 09:51 AM
#9
TECH Senior Member

Join Date: Feb 2009
Location: Detroit
Posts: 6,540

I'm not sure what you mean by EAP, but I am having a difficult time understanding what you're trying to say. I thought it was an interesting question, however I think the question itself is an oversimplification for a boosted engine. For a certain Comp Cams LS1 blower cam having an intake valve closing at 50° ABDC, apparent cylinder volume ends up being less than 90% of the total. With a finely tuned NA induction setup that reaches 100% VE somewhere near peak torque around 4000rpm, it would take 3 pounds of boost at sea level to get the same amount of air in the cylinder by the time the intake valve closes if you were making the same measurement at 0rpm. Even then, the piston is already traveling up the bore so the adiabatic pressure at TDC is even lower on the boosted example than the NA one. So the effective compression ratio that we've all heard in various shop talk circles really is nonsensical as the calculation doesn't produce a result that has any connection with what's happening in an engine. Again, if you're talking charge ratio, then it makes more sense...but you need more than just boost pressure and don't even need a compression ratio to figure that out.

Last edited by smokeshow; 06-19-2017 at 10:02 AM.

06-19-2017, 11:42 AM
#10

Join Date: Dec 2008
Location: Santa Ana, CA. USA
Posts: 2,075
Engine Analyzer Professional by Performance Trends

Hi S/S, thanks for the question and with respect to you, many other LS-1 Tech member, I will give a better explanation.

I use a program by Performance Trends called Engine Analyzer Pro.
I have used the "app" for over thirty five years.
I use this program to "predict" engine performance AND to produce a "build" sheet.

ALL here may "DEMO" this app for a period of time before purchase from PT. (free download)

NEXT S/S, yes I agree with a report of cylinder density.
THIS IS WAY I "spec" a Blower CAM with short inlet duration/MAX exhaust duration.
NOW your statement is OPPOSITE of correct cylinder pressure at TDC and other areas.
MY EAP "model" allows for cylinder pressure measurement at ALL cycle phases.
Examples:
1. The supercharged engine's piston has the MAP pressure (boost) "pushing" the piston down on the induction stroke, a good thing.
2. The TDC piston pressure is MAP/EMAP at "overlap" with respect to Wave Tuning Laws.
3. Density Ratio was NAMED by Bob Keller when at AiReserch (YOUR name of Charge Ratio) over forty years ago AND is accepted by Joe Moore (also AiResearch) my customer. (Smith Systems)
4.When the cylinder is supercharged it will be full of air/fuel (Non GDI) MUCH sooner that a N/A engine with requirement for SHORTER Inlet duration as LATE valve closing will only PUSH OUT the filled cylinder air into the inlet port.

THE BEST Dynamic C.R. "tech" is the Piston thread by DIAN and answered by Joe to SUPPORT your stance.

Lance

Last edited by pantera_efi; 06-19-2017 at 11:44 AM. Reason: SP

06-19-2017, 11:54 AM
#11
TECH Senior Member

Join Date: Feb 2009
Location: Detroit
Posts: 6,540

Tech is really going downhill these days, isn't it...

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